Master cylinders such as those commonly used in brake systems have had plastic brake fluid reservoirs mounted on the master cylinder bodies by means of mounting tubes integrally molded as part of the bottom of each reservoir. Each master cylinder body is provided with mounting bosses extending upwardly from the cylinder portion of the body. These bosses have recesses opening upwardly which receive the mounting tubes. Other master cylinders have used reservoir retaining fittings which are threaded or otherwise secured in the mounting boss recesses. Such fittings extend through openings in the bottom of the reservoir, and seals are provided between the bottom of the reservoir, the fittings, and the mounting bosses. Separate seals of various types have been employed to seal between each mounting tube and the wall of the recess into which it extends. Typical examples are shown in U.S. Pat. No. 4,309,937--Schardt; U.S. Pat. No. 4,373,333--Coleman; and U.S. Pat. No. 4,445,333--Coleman. In the Schardt patent, the retaining fittings are used, and O-ring seals are shown in recessed grooves at the tops of the mounting boss recesses. These O-ring seals are so located that they sealingly engage the bottom of the reservoir body and the retaining fittings as well as the mounting bosses. In the Coleman patents, the seals also fit in an interlocking manner with the mounting tubes and the mounting boss recesses so that they also retain the reservoir body on the master cylinder body.